1. Field of Invention
This invention relates to a method for the recovery of one or more trace elements from coal and more particularly to a method for the recovery of one or more trace elements, including gold and the platinum group elements, from coal and coal ash.
2. Background of the Invention
Coal is a solid heterogeneous composition including a mixture of pure hydrocarbons, oxygenated hydrocarbons and complex hydrocarbons including other elements, inorganic minerals and water. Differing geologic histories have resulted in the formation of coals with differing chemical properties.
The chemical properties of coal, are dependant largely on geologic origin and the conditions observed during formation of any given coal deposit. Additionally, the chemical properties of coal ash may be dependant on the parameters observed during combustion. The combustion of coal produces bottom ash and a combustion gas composed typically of nitrogen, carbon dioxide, water vapor, oxygen, and smaller amounts of inorganic elements and compounds, commonly in vapor state, and flyash.
In the combustion of coals, for instance in many power generating facilities, flyash accompanies combustion gas from the furnace through the flue combustion gas treatment and may be removed in baghouses or by precipitation. While flyash may include any naturally occurring element, aluminosilicate and calcium compounds typically account for more than 80% of the mass. Flyash commonly includes oxygenated inorganic compounds including silica, aluminum, calcium, magnesium, iron, sodium, potassium, titanium and sulfur as well as trace elements including mercury, silver, gold and various platinum group elements. As used herein, the term platinum group elements may include platinum, palladium, rhodium, ruthenium, osmium, and iridium. Ash may also accumulate in the bottom of the furnace as a loose, dry bottom ash or as a slag. Coal bottom ash tends to be a granular, porous material. Compared to fly ash, bottom ash is typically more coarse. Slag is typically formed as black glassy particles.
Considerable attention has been afforded the problems relating to removal of selected trace elements from flue gas, particularly sulfur and mercury. Additionally, at least one method has been developed for separating gold and platinum group elements from chloride containing solutions. U.S. Pat. No. 4,390,366 to Lea, et al., entitled Process for the Extraction of Precious Metals from Solutions Thereof, discloses a solvent extraction process for sequentially and selectively separating and removing gold and platinum group metal values from a chloride containing aqueous solution.
It may be desirable to provide a method for the recovery of one or more trace elements including gold and one or more platinum group elements from coal. More particularly, it may be desirable to provide a method which specifies parameters for the selection of coal, parameters for combustion of the pre-selected coal and the parameters for a process for recovering gold and one or more platinum group elements from the ash produced by the combustion of the identified coal.
Additionally, it may be desirable to provide a method for the recovery of one or more trace elements including gold and one or more platinum group elements from coal ash. More particularly, it may be desirable to provide a method which specifies parameters for the selection of coal ash and the parameters for a method for recovering silver, gold and one or more platinum group elements from the coal ash.
Therefore, it is one objective of the present invention to provide a method for the recovery of one or more trace elements including gold and one or more trace platinum group elements from coal and/or coal ash. More particularly, it is an objective of the present invention to provide a method which may specify parameters for the selection of coal and parameters for combustion of the selected coal, or in the alternative, the parameters for the selection of a coal ash, as well as the parameters for the recovery of gold and one or more trace platinum group elements from the ash produced by the combustion of coal and/or coal ash.
The present invention is directed to a method for the recovery of one or more trace elements including gold and one or more platinum group elements from coal. As used herein the term coal may include both coal and lignite. The present invention is also directed to a method for the preparation of a dore bar including gold and one or more platinum group elements. Additionally, the present invention is directed to a method for the preparation of an anode slime including gold and one or more platinum group elements. The present invention specifies parameters for the preparation and mixing of a charge for a furnace including ash from the combustion of a selected coal, an inquart and a fluxing agent, the parameters for the heating of the charge and the casting of a dore bar. The present invention may also specify parameters for the preparation of an anode slime including gold and one or more trace platinum group elements from the ash produced by the combustion of the pre-selected coal. Additionally, the present invention may also specify one or more parameters for the identification and selection of coal including gold and one or more platinum group elements, one or more parameters for the combustion of a pre-selected coal producing an ash for mixing to form a charge, the ash including gold and one or more platinum group elements.
According to one embodiment, the method for the recovery of one or more trace elements from coal includes the steps of mixing a charge including an ash formed by the combustion of a pre-selected coal, the ash including gold and one or more platinum group elements, an inquart including gold and one or more platinum group elements and a fluxing agent, charging the furnace with the charge, heating the charge in the furnace to a pre-selected temperature forming a molten charge including a molten inquart, absorbing a portion of the gold and one or more platinum group elements of the ash into the molten inquart, discharging a molten slag from the furnace and casting a dore bar by discharging the molten inquart including an absorbed portion of the gold and one or more platinum group elements of the ash to a mold.
An alternate preferred embodiment of the invention also includes the additional steps of digesting the dore bar in an electrowinning process wherein the dore bar is introduced into an acidic bath as an anode and an electrical potential is maintained between the dore bar and a cathode plate, reducing silver ions onto the cathode plate and precipitating and dissolving trace elements in the electrolytic solution producing an anode slime.
An alternate preferred embodiment of the invention also includes the additional steps of recovering gold and one or more platinum group elements from the anode slimes by any of a variety of means. One such method calls for the digestion of the anode slimes using a 30% solution of HNO3. The digested solution is filtered and rinsed. The liquid rinse portion contains silver in solution and may be washed with HCl and filtered. The residue of the filtration of the digested slimes is mixed with aqua regia forming a solution containing gold, platinum and palladium. Gold may be removed by solvent extraction, or in the alternative gold may be removed by treatment with iron chloride, (FeCl2). Rhodium and other trace elements may be removed by precipitation by treatment of the solution with potassium hydroxide and sodium borohydride. Palladium in solution may be removed by any of a variety of known methods including precipitation by treatment with ammonium hydroxide, NH4OH, and HCl as the complex PdCl2(NH3)2. This yields palladium metal by burning. Alternately, palladium in solution may be removed by precipitation with DiMethyl Glyoxime, followed by filtration and reduction and burning or ashing of the residue. Platinum in solution may be removed by any of a variety of known methods including precipitation as impure (NH4)2PtCl6 on treatment with NH4Cl, leaving H2PdCl4 in solution. In the event that an organic solvent such as DiMethyl Glyoxime is employed to extract gold, and, prior to treatment with NH4Cl, the solution should be treated with ethanol and hydrogen peroxide. The solution may be filtered and the residue, including (NH4)2PtCl6, subjected to reduction and burning leaving an impure platinum sponge. The platinum sponge may be purified by redissolving in aqua regia. Alternately, platinum may be recovered by precipitation of pure (NH4)2PtCl6 by treatment with ammonium hydroxide, NH4OH. This yields platinum metal by burning.
The preferred embodiment of the invention includes the additional steps of pre-selecting a coal for combustion including gold and one or more platinum group elements for combustion and combusting the pre-selected coal including gold and one or more platinum group elements producing an ash including gold and one or more platinum group elements. To date it has been demonstrated that, the Blackhawk formation located in the state of Utah, the Powder River coal field located in Wyoming, and the Yegua and Manning formations, produce coal that, when combusted produce an ash including acceptable levels of gold and one or more platinum group elements recoverable by the method of the present invention.
Additionally, it is believed that coals may be identified by chemical composition which, upon combustion may produce an ash having levels of gold and one or more trace platinum group elements which may be of sufficient concentrations and present in a form recoverable by the method of the present invention, such as to justify the costs of extraction. More particularly, it has been observed that coals that include a bromine concentration of less than or equal to 1.00 ppm, exhibit a tendency to release or otherwise permit the absorption of gold and platinum group elements by a molten inquart of a molten charge, thereby rendering the gold and platinum group elements more readily recoverable than these elements appear to be in coals having higher concentrations of bromine. Alternately, it has been observed that coals that include a bromine concentration of less than or equal to 76.00 ppm and which exhibit concentrations of chlorine, aluminum and one or more transition elements that are within specified ranges may produce an ash having levels of gold and one or more trace platinum group elements which may be of sufficient concentrations and present in a form recoverable by the method of the present invention, in quantities such as to justify the costs of extraction.
Without being bound by theory, it is believed that due to the relatively lower concentrations of bromine, chlorine, aluminum and one or more transition elements in identifiable coals, including Blackhawk coal, Powder River coal and Texas lignite, a lower frequency or concentration of compounds including coordination compounds, chelates and/or complexes are formed in the coal and or the ash produced by the combustion of such coals, which may tie up or bind trace gold and platinum group elements. Hence, it is believed that in identifiable coals, including coals extracted from the Blackhawk formation, the Powder River coal field and lignite coals from the Yegua and Manning lignite formations found in Texas, include levels of gold and one or more trace platinum group elements which are of sufficient concentrations and present in a form recoverable by the method of the present invention, in quantities such as to justify the costs of extraction.
Additionally, and without being bound by theory, it is believed that the combustion of other coals from other formations may be identified by empirical, analytical or statistical means, which may result in the production of a flyash containing gold and one or more trace platinum group elements that are recovered by the method of the present invention. It is the intention that the scope of the present invention specifically include both the extraction of trace gold and platinum group elements from ash produced by the combustion of Blackhawk coal as well as from ash produced by the combustion of coal from other formations that may be identified by such means and which result in the production of a flyash containing concentrations of gold and one or more trace platinum group elements which are of sufficient concentrations and present in a form recoverable by the method of the present invention, in quantities such as to justify the costs of extraction.
The present invention consists of the method and system hereinafter more fully described, illustrated in the accompanying drawings and more particularly pointed out in the appended claims, it being understood that changes may be made in the form, size, scale, proportions and other details of construction without departing from the spirit or sacrificing any of the advantages of the invention.